Preliminary reconstructions of the North Atlantic Oscillation and the Southern Oscillation index from measures of wind strength and direction taken during the CLIWOC period

Using measures of wind strength and direction taken onboard ships during the 1750-1850 (CLIWOC project) period, preliminary reconstructions are attempted for the North Atlantic Oscillation (NAO) and the Southern Oscillation Index (SOI). The reconstructions are based on regression equations developed using similar data from the ICOADS dataset. Although the regression relationships developed over a calibration period (1881-1940) work almost as well over an independent verification period (1941-1997), application to the earlier 1750-1850 period results in barely statistically significant correlation coefficients when compared with a number of other NAO and SOI reconstructions from other proxy and long instrumental sources. A number of possibilities are investigated to attempt to determine the cause, the most likely of which is that the number of observations available for the CLIWOC period is just too low in some regions. As large numbers of ships' logbooks remain to be digitised, the regression relationships will prove useful to focus effort in future digitisation endeavours.